The invention relates to a reactor pressure vessel with an upper support plate, which occupies its cross section above upper connecting branches and divides an upper dome chamber from a lower chamber. The support plate has openings to accommodate control rods and at least one equalization opening (dome bypass).
The invention also relates to a process for temperature equalization between an upper dome chamber above and a lower chamber below a support plate in a reactor pressure vessel of a reactor plant. The support plate occupying the cross section of the reactor pressure vessel above the upper connecting branches, and a medium flowing through an equalization opening in the lattice plate.
In a nuclear power station with a pressurized water reactor an upper support plate, which occupies the cross section of the reactor pressure vessel, is generally disposed in the reactor pressure vessel above the upper connecting branches of the primary circulation. The plate divides the reactor pressure vessel into a larger lower chamber and a smaller upper dome chamber facing the pressure vessel dome and serves to hold control rods in position.
At least two different configuration variants exist. The first variant is used mainly in Germany. It provides for the upper support plate to be configured as a lattice plate. In such a lattice plate the openings for the control rods are each significantly larger than the cross section of a control rod. This therefore leaves a relatively large free cross section surrounding the control rod in each opening. Through this free cross section an exchange of medium (coolant exchange) is possible between the upper dome chamber and the lower chamber in the reactor pressure vessel. An additional opening (dome bypass) nevertheless exists in the edge area of the upper lattice plate. This configuration is referred to as a xe2x80x9chot domexe2x80x9d.
The second configuration variant for a reactor pressure vessel, which is used mainly in France and the USA, provides for openings in the support plate to accommodate control rods, in which the cross section of an opening largely corresponds to the control rod cross section. Very little space, if any, therefore remains for an exchange of medium (coolant exchange) between the two chambers adjoining the support plate. This variant is referred to as xe2x80x9ccold domexe2x80x9d and requires a relatively large equalization opening (dome bypass) in the edge area of the support plate in order to ensure a sufficient exchange of coolant at all times.
During continuous power output operation of a nuclear power station a small dome bypass with a small cross section is sufficient. The small bypass must merely be capable of ensuring that a homogeneous boron concentration is maintained throughout the reactor pressure vessel. Too large a dome bypass is undesirable in power output operation, since this would result in the coolant circulation in the reactor pressure vessel being led not only through the reactor core but also through the upper dome chamber, which would lead to a reduction of the reactor outlet temperature of the coolant and to an increase in the necessary capacity of the coolant pump in the primary circulation and hence to a power output loss of the power station.
A reactor pressure vessel of the xe2x80x9ccold domexe2x80x9d variant largely keeps the hot coolant away from the dome chamber, since the control rods are fitted as tightly as possible into openings provided for them in the support plate. The above-mentioned power output loss occurs, however, if a relatively large dome bypass is provided.
The variant of the reactor pressure vessel referred to as xe2x80x9chot domexe2x80x9d has large openings in the lattice plate, large parts of which remain free even with the control rods are inserted. Because the coolant circulation in the reactor pressure vessel is also always led through the upper dome chamber, in power output operation the temperature of the coolant in the dome chamber is consequently largely identical to the temperature below the support plate.
Another situation results when shutting a nuclear power station down for an inspection or for changing fuel elements. In both variants of the reactor pressure vessel, the coolant present in the reactor pressure vessel must then be cooled from approximately 300xc2x0 C. to approximately 50xc2x0 C. The cooling time for the content of the dome chamber and the reactor dome itself is determined by the size of the dome bypass.
In the xe2x80x9ccold domexe2x80x9d variant a relatively large dome bypass would be needed in order to obtain a sufficiently short cooling time. As stated, however, such a dome bypass would cause a high power station output loss.
Although a coolant exchange through the lattice plate is in principle possible in the case of the xe2x80x9chot domexe2x80x9d variant, when running the plant down, that is to say when cooling by way of the coolant circulation, the hot coolant of lower relative density above the connecting branches remains in the upper dome chamber, while the temperature in the lower part of the reactor pressure vessel continually falls.
The temperature difference between the lower part of the reactor pressure vessel and its dome that occurs when shutting the nuclear power station down results in different thermal expansions of the two parts. This may lead to the dome bolts being somewhat skewed in their threads so that they cannot be immediately screwed out. An additional cooling time is therefore necessary, so that the reactor dome can also assume the temperature of the rest of the reactor pressure vessel.
It is accordingly an object of the invention to provide a reactor pressure vessel and a process for temperature equalization in the reactor pressure vessel that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, in which a minimum possible loss is to be sustained in power output operation, and in which temperature differences between the lower part of the reactor pressure vessel and the upper reactor dome do not occur or are rapidly equalized when running down the nuclear power station.
With the foregoing and other objects in view there is provided, in accordance with the invention, a reactor pressure vessel. The reactor pressure vessel contains a reactor pressure body having a chamber formed therein and a cross-section, and upper connecting branches connected to the reactor pressure body. An upper support plate is disposed in the reactor pressure body above the upper connecting branches and expanding over the cross-section of the reactor pressure body. The upper support plate divides the chamber of the reactor pressure body into an upper dome chamber and a lower chamber. The upper support plate has openings formed therein for accommodating control rods and at least one equalization opening formed therein, the equalization opening has a cross section being variable as an inverse function of a temperature in the lower chamber.
Another object of the invention is to specify a suitable process for temperature equalization between the upper dome chamber and the lower chamber in the reactor pressure vessel, situated above and below the support plate respectively.
According to the invention the first aforementioned object is achieved in that the cross section of the equalization opening (dome bypass) is variable as an inverse function of the temperature in the lower chamber.
This affords the advantage that in power output operation of the nuclear power station with a small cross section of the equalization opening (dome bypass) only a little medium (coolant) passes into the dome chamber, whereas when running or shutting the nuclear power station down with a large cross section of the equalization opening (of the dome bypass) a rapid exchange of medium and hence temperature exchange occurs between the upper dome chamber and the lower chamber of the reactor pressure vessel.
Power losses otherwise attributable to a large dome bypass are advantageously minimized. It is consequently possible to manage with a correspondingly small coolant pump capacity for the primary circulation. Just enough coolant is exchanged between the chambers adjoining the support plate to keep the boron concentration on both sides of the support plate largely equal, which is advisable for routine operation.
The reactor pressure vessel according to the invention affords the advantage when running a nuclear power station down that the reactor dome is cooled with a similar rapidity as the rest of the reactor pressure vessel, owing to the then significantly increased exchange of coolant through the equalization opening (dome bypass). The still hot coolant, which is either trapped above a largely closed support plate or, where a lattice plate is provided, has collected beneath the reactor dome solely due to the buoyancy of the hotter medium in the colder medium, is replaced by cooler medium through the enlarged cross section of the equalization opening (dome bypass). This results therefore in a more uniform cooling of the lower part of the reactor pressure vessel and of the upper reactor dome than hitherto, so that no additional cooling times are required in order to be able to remove the reactor dome.
For example, the equalization opening forms a dome bypass between the lower chamber, which is a reactor annulus defined by the pressure vessel wall and the core installations, and the dome chamber. The connecting branches open into the annulus, known in the art, so that the coolant can advantageously pass into the upper dome chamber by a short route, especially through the dome bypass. From there it flows either along the control rods or through the openings in the lattice plate back into the lower part of the reactor pressure vessel. In the xe2x80x9ccold domexe2x80x9d variant the control rods, for example, are surrounded by cladding tubes, in which space remains for a slight through-flow of coolant.
For example, the cross section of the equalization opening (dome bypass) is passively temperature-controlled. This affords the particular advantage that without external intervention the available opening is at all times suited to a prevailing temperature.
For example, a device is fitted in the equalization opening (dome bypass), which device in the event of a temperature drop enlarges the cross section of the equalization opening by utilizing the thermal contraction of the material of the device, and in the event of a temperature rise reduces the cross section of the equalization opening by utilizing the thermal expansion of the material of the device. Such a device is especially suited to passively controlling the equalization opening as a function of the temperature.
The equalization opening (dome bypass) contains an expansion sleeve, for example, which enlarges the flow cross section in the event of a temperature drop. Such an expansion sleeve, is disclosed by Published, Non-Prosecuted German Patent Application DE 196 07 693 A1, and varies the cross section of an opening solely on the basis of the prevailing temperature.
The expansion sleeve is connected, for example, to a hollow piston guided in a cylinder open at an end. The inside of the piston is connected by way of end openings of the piston both to the lower chamber below the support plate and to the dome chamber above it. The piston also has additional lateral openings, which when the expansion sleeve contracts align with openings situated in the side wall of the cylinder, or overlap the openings. The cross section of the equalization opening (dome bypass) is enlarged when the expansion sleeve contracts due to a drop in temperature.
According to the invention the object of specifying a suitable process for temperature equalization is achieved in that the flow of medium is varied as an inverse function of the temperature in the lower chamber.
This affords the advantage that in power output operation of the nuclear power station a reduction in the power output of the power station cannot occur due to a low medium flow (coolant flow) at high temperature. Conversely, when a rapid cooling of the medium (coolant) in the upper dome chamber is required when running the power station down, the desired temperature drop in the upper dome chamber is achieved in a short time owing to a high medium flow (coolant flow) at relatively low temperature.
For example, a bypass flow of the medium through the equalization opening connects the lower chamber, which may be a reactor annulus defined by the pressure vessel wall and the core installations, to the upper dome chamber. In this way a rapid and reliable feeding of the medium (coolant) into the upper dome chamber is possible when necessary. The medium can leave the upper dome chamber along the control rods.
The medium flow (coolant flow) is passively temperature-controlled, for example. No expensive additional devices are then needed to control the flow.
The medium flow is increased, for example, owing to a temperature drop in the reactor pressure vessel when running the reactor plant down. This affords the advantage that just when running down the nuclear power station with a resultant cooling of the lower chamber of the reactor pressure vessel, a rapid temperature equalization is ensured with the upper dome chamber in the reactor pressure vessel.
The reactor pressure vessel and the process according to the invention in particular afford the advantage that the cross section of the equalization opening (dome bypass) in the upper support plate in the reactor pressure vessel forming a bypass and the strength of the medium flow in the bypass at all times correspond to the requirements.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a reactor pressure vessel and a process for temperature equalization in a reactor pressure vessel, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.